Electro-aerodynamic generator

ABSTRACT

This invention relates to an electrodynamic generator of the type comprising a passage for the flow of a fluid at least partially at supersonic speed from an upstream portion constituted by a convergent-divergent region in which is disposed ionization means substantially in the neck of the said region, to a downstream region at high potential in the neighborhood of the outlet from the passage. The invention is characterized in that the passage is constituted of a ceramic material or vitrified material and in that the internal wall is very smooth. Moreover, filters are provided in order to obtain an air supply substantially free from impurities. Such a generator which provides low currents at high voltages (several tens of kilovolts) is particularly suitable for the electrostatic deposition of coating products.

United States Patent Tholome i541 ELECTRO-AERODYNAMIC' v GENERATOR [72]Inventor: Roger Tholome, 38 Meylan, France [73] Assignee: Tunzinl-Sames,Grenoble, France [22] Filed: July 31, 1970.

[21] App1.No.: 59,910

[52] US. Cl ..310/10, 310/6 [51] Int. Cl. ..1102n 3/00 [58] Field ofSearch ..310/5, 6, 10

[56] References Cited UNITED STATES PATENTS 3,439,197 7 4/1969 Ohain etal..' ..310/ll 2,308,884 1/1943 Lindenblad ..310/5 3,225,225 12/1965Wattendorf et al. ....310/6 3,417,267 12/1968 Marks ..3l0/6 PrimaryExaminer-D. X. Sliney Aztorney-l-lolcombe, Wetherill and BriseboisABSTRACT This invention relates to an electrodynamic generator of thetype comprising a passage for the flow of a fluid at least partially atsupersonic speed from an upstream portion constituted by aconvergent-divergent region in which is disposed ionization meanssubstantially in the neck of the said region, to a downstream region athigh potential in the neighborhood of the outlet from the passage. Theinvention is, characterized in that the passage is constituted of aceramic material or'vitrified material and in that the internal wall isvery smooth. Moreover, filters are provided in order to obtain an airsupply substantially free from impurities. Such a generator whichprovides low currents at high voltages (several tens of kilovolt s) isparticularly suitable for the electrostatic deposition of coatingproducts.

3 Claims, 2 Drawing Figures PATENTEUnEc 19 I972 Ixweni-or 7 lELECTRO-AERODYNAMIC GENERATOR The present invention relates to anelectro-aero dynamic generator of the type comprising a passage for theflow of a fluid, at least partially at supersonic speed, from anupstreamportion, constituted by a convergentdivergent region havingionization means disposed substantially in the'neck of said region, to adownstream zone at high potential in the neighborhood of the outlet fromthe said passage. Such generators effect the delivery of electricalenergy at high potential by converting a part of the thermodynamicenergy of a gas, for example air, into electrical energy by making itgoup in an electric field with electrical charges transported by thisair. The charge current transported is, to a certain degree, a directfunction of the speed of the air, and it is therefore advantageous thatthe speed of flow of the gaseous fluid is as high as possible. This iswhy it is advantageous to use supersonic speeds.

Experience has shown that such generators will permit the formation ofsupersonic currents, but that the supersonic flow presents certainanomalies. One of the causes of these anomalies is constituted bydeposits of viscous produces on the intemal wall of the flow passagelocated downstream of the convergent divergent region, such that theycreate turbulence and possibly local short circuits leading toconsiderable reductions of the output voltage of such a generator.

The present invention has for an object to reduce these disadvantagesand to provide a generator which effects the formation of a particularlyhomogenous flow, substantially without turbulence and this duringperiods of operation which are particularly prolonged.

According to a first characteristic of the invention, the flow passagefor the supersonic fluid disposed downstream of the convergent-divergentregion is constituted by a ceramic material or a vitrified mineral andits internal wall is smooth. Equally so far as concerns the internalwall of the convergent-divergent region, the latter must be produced ofa material enabling one to obtain a very smooth surface and moreoverresistant to electrical erosion.

According to another feature of the invention, the generator is suppliedwith air substantially free from dust and hydrocarbons, but however witha hygro-metrical degree greater than 50 percent. These arrangementsenable the production of generators whose performances and durations ofoperation are clearly superior than those of known types of electro-aerodynamic generators.

The invention will now be further described by way of example withreference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic sectional view of one embodiment of generatoraccording to the invention, and

FIG. 2 is a" sectional view showing a detail of the generator in FIG. 1.

Referring to the drawings the generator essentially comprises a flowpassage 1 having at its upstream end a convergent-divergent region 2 ofwhich the internal wall is made of metal and connected to earthpotential. An ionization needle 3 is positioned in the neck of the saidconvergent-divergent region. This region 2 leads to a coaxial passage 4having a length several times greater than that of the region 2. Forexample the convergent-divergent region 2 may have a length of the orderof one centimeter while the insulating passage 4 has a length of theorder of 10 centimeters. With this arrangement there can be obtainedfrom an excitation potential of the order of 4 to 5 kV fed over thecable 5 and with a flow at least partially'supersonic in the interior ofthe passage 4, an output potential of the order of 40 to kV at a currentof several tens of micro amperes. The air is admitted to the generatorthrough a pipe 6 in which are disposed a filter 7 and apre-filter 8 witha drain 9 for the evacuation of condensates at 10.

In order to obtain a suitable flow within the passage 1 of the generatorand particularly a flow at least partially supersonic at the interior ofthe passage 4, it is preferable to employ the following arrangements. Sofar as concerns the passage 4 for the flow .at least partially atsupersonic speed which is necessarily constructed of an insulatingmaterial and of which the diameter is of the order of some millimetersonly, it must be ensured that the internalwall of this passage isextremely smooth. If a very high degree of polish is not obtained, thenthe supersonic flow cannot be entirely assured. Amongst the materialscapable of ensuring this polish, it is necessary to eliminate thosewhich are not capable of presenting a resistance to electrical erosion,particularly oxidation to the intense ionization which exists within thepassage. It has been determined that ceramic materials or other mineralmaterials can be used with advantage. In order to obtain a satisfactorypolish, it is preferred to use such'vitrified materials, for example rawtubes of molten glass (rough fused glass) particularly with 'a highsilicon content, such as for example those produced under the trade namePYREX without any subsequent polishing.

So far as concerns the convergenbdivergent region, this must be producedof a conductive material presenting a high resistance to oxidation byreason of the presence of ozone given off in the ionization discharge atthe region of the needle 3. This material must moreover be subjected toa very fine polish in order to disturb as little as possible the highspeed flow. For example, brass has a sufficient polish, is rapidlydegraded by ionization, while stainless steel of the 18/8 type betterresists oxidation, but is difficult to machine, thus making thepolishing operation long and costly. The use of a material which caneasily be machined and polished, such as cut-out brass, covered withan'alloy resistant to corrosion, such as chromium, seems to provide agood solution, but the electrolytic coating at the interior of a holewhich is of the order of 2 millimeters diameter and 5 millimeters inlength necessitates an electrode and it is a very delicate industrialoperation to produce. it has been shown that a convenient and economicprocedure is' to efiect a chemical nickel plating, without employing anelectric current, according to the process known as the Kanigen process,which enables a very fine deposit to be obtained of a nickelphosphorusalloy by the simple immersion in a bath of nickel salts. The depositthus obtained which is of a very uniform thickness, with differences ofthickness less than 10 percent, presents a good hardness greater than500 vickers and its structure is completely amorphous and presents anabsence of porosity, even for a deposit of the order of some 10 micronsthick. The uniformity and homogeneity of the deposit enables theobtention of a particularly polished surface state by 1 nnnr I "(00 asimple brightening. This method of production which utilizes thechemical coating according to the Kanigen process presents'the advantagethat it does not employ an electric current, such that the support ofthe nickelphosphorus coating, according to the process, may beconstituted by an insulating material such as a thermoplastic orthermosetting material, which enables simplification and to reduce theprice of producing the support. It is thought that another economicprocess would be the metallization by vaporization in a vacuum of ametal presenting the required anti-corrosive qualities (such as gold,silver, nickel), the support being either insulating or metallic. Thereis shown in FIG. 2 a composite nozzle 2 forming the convergent-divergentregion and constituted by an insulating support 2a and a metalliccovering 2b.

In the case of the convergent-divergent nozzle, or the supersonic flowpassage, it has been shown that it is convenient to retain on all sidesof a certain roughness and by way of example, there are given thefollowing experimental results. Two measurements were made, one of thetotal roughness (RT) which is the difference between the highest peakand the deepest hollow along a particular length of the passage, whileone measures the arithmetic difference (RA) which is the arithmetic meanof the absolute value of the differences from a neutral line along thissame passage, and it has been shown that with a convergent-divergentnozzle of brass covered with a nickel-phosphorus alloy according to theKanigen process and with a final brightening, and also with a passageformed of a tube of Pyrex, the total roughness (RT) is between 0.5 and0.7 microns, while the mean difference (RA) is 0.09 microns, while thesesame components, but made of polished stainless steel for the nozzle anda plastics tube for the passage, according to usual techniques, presenta roughness about ten times greater and performances, from the point ofview of electrical energy obtained at the output, of the order of to 40%less than that of the nozzle and passage according to the invention.Another improvement according to the invention relates to the means offeedingthe gaseous fluid to an electro-aero dynamic generator and-thisis shown in FIG. 1. In effect, the compressed air in the passage 3 whichis generally used to convey charges is provided by pistontypecompressors which are lubricated with oil and this air generallycontains, amongst other impurities such as metal or metal oxideparticles, traces of oil which is more or less oxidized. Theseimpurities create in the nozzle 2 and in the passage 4 deposits which,in the long term are prejudicial not only to the flow of gaseous fluidbut which may lead also to the establishment of leakage currents in thepassage and a large decrease in the output potential, as well as areduced flow of electric current. It has been shown that it isconvenient therefore to feed such generators with very pure air and onehas been led to provide, upstream of the generator, filters 8 and 7which are particularly efficient, such as absolus filters (reference 8).These absolus filters enable the trapping of particles in a proportionof 99.98 percent of the particles having a diameter greater than 0.3microns. Such filters are therefore generally indispensable and maysometimes be sufficient. However, by reason of the increase oftemperature resulting from the compression of the air in the compressorwhich is situated upstream of the filter 7 and 8, it can produce localvaporizations of the hydrocarbons which are not stopped by the filter 8,even if it is of excellent quality, so that these vapors penetrate intothe generator, where the air, subjected to a drop, has its temperatureconsiderably reduced. These hydrocarbon vapors may then be condensedwithin the generatorand may be precipated on the walls by ionizationaction. Thus, even with a very fine filter, the operating conditions ofthe generator are such that it is very'difficult to stop the hydrocarbonvapors and the invention recognizes therefore to abstain from feedingthe generator with air containing condensable oil vapors. To this endthe invention provides the use, in compressors, of lubricating oil witha very low vapor tension and in all cases free of all volatilecomponents such as certain additives or anti-oxidizing agents.

Advantageously there is placed between the compressor and the generatora filter containinga substance on which hydrocarbon molecules have a tendency to deposit or be absorbed. These substances are advantageouslyused in a form offering a very large contactsurface with the gas to bepurified, for example in granular or pulverulent form. But it is alsoconvenient to choose, amongst the numerous substances which are capableof stopping oil vapors, those which have the smallest affinity withwater, for on one hand there would be rapid saturation by water of thefilters and on the other hand it is convenient to maintain a hygrometricdegree suitable in the air, since the operation of the generatorinvolves the formation of small particles of condensed water, whichserve in fact to carry the electric charges within the generator.Amongst these products one may mention activated carbons.

By way of example a generator of 60 kV and of 10 micro-amperes consuming13 N cubic meters per hour, under an absolute pressure'of 5 bars, fedwith air by an industrial compressor positioned upstream of an absolufilter of the type described above may function about a hundred hoursuntil the moment where the walls of the passage are covered with aviscous deposit of hydrocarbons either oxidized and/or non-oxidized.This deposit makes the voltage and current of the generator drop toone-fifth of their initial values. The same generator connecteddownstream of a cartridge containing 200 grams of active carbon doesnothave its performance substantially reduced after 1,300 hours ofoperation, the passage having practically no deposit.

During this time of operation, the filter cartridge which has beentraversed by 17,000 Nm of air containing.

several grams of water by Nm (that is to say a total of several tens ofkilograms of water) is only increased by ll grams, almost entirelyconstituted by hydrocarbons or hydrocarbon derivatives.

I claim:

1. In a device comprising an electro-aerodynamic generator of the typehaving a passage for the flowof a fluid at least partially at supersonicspeed from an upstream region constituted by a convergent-divergentregion with ionization means substantially in the neck of said region,to a downstream zone at high potential in the neighborhood of the outletfrom the said passage, the improvement which comprises a very finefilter positioned upstream of said generator and stopping at l060l l0600 UNiTE-D STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.-.3,706,89 Dated December 19, 1972 Inventor(s) ROGER THOLOME It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

[30] Foreign Application Priority Data July 31, 1969 France 69-26381Signed and sealed this 8th day of May 1973.

(SEAL) Attest:

EDE-JARD ELFLETCHERJR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents FORM 5 0-1050 (10-69) USCOMM-DC 6O376-F'69 us. GOVERNMENTPRINTING OFFICE: I969 0-366-334

1. In a device comprising an electro-aerodynamic generator of the typehaving a passage for the flow of a fluid at least partially atsupersonic speed from an upstream region constituted by aconvergent-divergent region with ionization means substantially in theneck of said region, to a downstream zone at high potential in theneighborhood of the outlet from the said passage, the improvement whichcomprises a very fine filter positioned upstream of said generator andstopping at least 99 percent of those particles of the fluid having adiameter greater than 0.3 microns.
 2. Device according to claim 1, inwhich said filter contains substantially non-hygroscopic absorbants forhydrocarbons.
 3. Device according to claim 2, in which said absorbantsare superactive carbons.